A full investigation aimed at identifying the exact causes is expected by end-March, French Health Minister Marisol Touraine said on Thursday.

No regulations were breached and it was impossible at this stage to establish exactly what went wrong, Touraine said.

"The tests were carried out in compliance with current regulations," she said.

The victims had been given an experimental drug made by Portuguese company Bial in an initial phase 1 stage.

Biotrial , the laboratory in charge of conducting the tests in a private facility in Rennes, Brittany, said in a statement it had strictly complied to international standards. Nobody at Bial was immediately available for comment.

All trials on the drug, which is intended to treat mood and anxiety issues as well as movement coordination disorders linked to neurological issues, have since been suspended.

Touraine said Biotrial should have halted the tests right after the first person was hospitalised, but instead five more people were given the medicine the next day.

The lab also should have warned authorities promptly about the accident and explicitly asked other participants whether they wanted to stay in the test trial, she added, presenting the main findings in the investigators' initial report.

Touraine said the first accident was on a Sunday evening, yet authorities were not alerted until the following Thursday.

Touraine said investigators had not called for suspension of Biotrial's authorisations to carry out drug trials.

The medicine involved is a so-called FAAH inhibitor that works by targeting the body's endocannabinoid system, which is also responsible for the human response to cannabis, but is not a cannabis-derivated drug.

Asked if the man who died last month had ingested cannabis before taking the drug, Touraine said it was possible.

In total, 90 people participated in the trial and the six men who fell ill had been in good health until taking the oral medication at the Biotrial facility.

Touraine said the medical condition of the five volunteers still in hospital was improving but that it was premature to envisage their complete recovery.

Cases of early-stage clinical trials going badly wrong are rare but not unheard of. In 2006, six healthy volunteers given an experimental drug in London ended up in intensive care.

On Jan. 21, U.S. Johnson & Johnson said it had suspended international trials of a drug similar to the one experimented by Bial.